Piston rings for internal combustion engines should have a gas sealing function and a lubricant controlling function. Therefore, in general, two piston rings for gas seal and one piston ring for lubricant control are used in combination. Of these piston rings, the gas seal piston ring is greatly affected by combustion gas because it is arranged immediately below the top land of the piston. More specifically, the gas seal piston ring is strongly pushed against the cylinder liner wall not only by its own tension but also by the gas pressure applied to the upper surface and the inner cylindrical wall of the piston ring, thus providing a great frictional force.
An abnormal wearing phenomenon called "scuffing" is liable to take place with the gas seal piston ring to which lubricant is sparingly supplied. Such a gas seal piston ring is, in general, made of cast iron, and has its outer peripheral surface plated with chrominum in order to improve the wear resistance. It suffers from the difficulty that when it is used under severe conditions such as high temperature, high load, and high corrosion in a diesel engine or a high lead gasoline engine, the chromium deposition on the outer peripheral surface of the piston ring peels off due to fatigue, becomes corroded, or is worn out. In some cases the entire ring is thermally collapsed.
Japanese Patent Application Laid-Open No. 203848/1982, proposes a steel gas seal piston ring having a soft nitrided layer on its surface which is suitable for use under severe conditions. The steel gas seal piston ring is advantageous in several points; that is, it is high in thermal collapse resistance, corrosion resistance, and wear resistance. It has been found that the piston ring is still disadvantageous, however, because its surface roughness is large and it does not conform well to the cylinder during initial operation. Accordingly, scuffing is liable to take place.